You are saying that if they change a "3" into a "4" they are diluting the brand?
Jezus christ people, you are all brainwashed by marketing. Seriously.

Either you believe in "BMW M", and you don't care what name or number they put on a car, as long as it lives up to the M standards.
Or you just want to brag to your friends with your M3 badge.

Of course we all got brainwashed by marketing at certain level. Otherwise there is no need for the M brand car. Why not randomly call the car 340i instead of M3?

The M3 and some other M cars like M1, M5 make the M division so popular and successful and then this “M” brand makes the other M cars so popular and successful. Now BMW is going to cut this core brand - M3 into half by sharing with M4. This will certainly affect the brand image.

We do not choose to drive M3 to brag friends or other people. When I drive my M3 and get appreciation from people on the street, I feel good because I know that they have good knowledge of car. Otherwise, they just think that this is just another German make Coupe or Sedan! If I drove Porsche or Ferrari and got appreciation, I bet 90% of them just thought that I can afford a dam expensive sport car.

I can accept some car lovers come to the forum and claim that they do not care about the M3, M4 branding. There are people who change car (model and brand) like changing girlfriends any way. However, there are some die hard BMW enthusiasts that the M3 branding is matter to them and you need to accept this as well.